"I can't remember" sounds like a single kind of failure, but cognitive psychologists treat long-term memory as a three-stage process, and a breakdown can happen at any one of those stages for entirely different reasons. Something you experienced might never have made it into memory properly in the first place, might have been stored but decayed or become inaccessible over time, or might still be sitting there intact and simply unreachable without the right prompt to bring it back — three genuinely different failure modes, often collapsed under the single vague complaint of "forgetting."
Encoding: getting information in requires attention
Encoding is the process of converting a perceived experience into a form memory can actually retain, and it depends heavily on how much attention was directed at the experience in the first place. An ordinary Tuesday breakfast typically receives minimal conscious attention — it's routine, unremarkable, and processed largely on autopilot — which means it's often never encoded into long-term memory with any real durability to begin with. This is why you can eat breakfast every single day and still be unable to specifically recall what you had on a particular unremarkable morning weeks ago: there was likely never a distinct, deeply encoded memory of that specific breakfast to lose in the first place, as opposed to a memory that existed and was later forgotten.
Storage decay and retrieval failure are two different problems
Even information that is successfully encoded can be lost through storage decay — the gradual weakening of a memory trace over time, particularly if it isn't reinforced through rehearsal or repeated retrieval. But a memory can also remain genuinely intact in storage and still fail to surface, a distinct problem called retrieval failure, where the information exists but the right cue or context needed to access it isn't present at the moment you're trying to recall it — which is exactly why a seemingly forgotten memory can suddenly resurface, vividly, when triggered by an unexpected smell, a familiar location, or some other cue that happens to match the context in which the memory was originally encoded. Distinguishing which of these three stages actually failed for a given lost memory isn't always possible from the inside; a memory that "feels" completely gone might really be an unreachable one waiting for the right cue rather than one that was never stored at all.
What we're still unsure about
The basic three-stage framework of encoding, storage, and retrieval, and the distinct mechanisms behind decay and retrieval failure, are well supported by decades of cognitive psychology research. What remains more genuinely difficult to pin down is distinguishing these failure modes definitively in any single specific case of forgetting — since a person experiencing a memory failure generally has no direct access to whether the information was never encoded, has decayed in storage, or is simply unreachable at that moment, researchers largely have to infer which mechanism is at work indirectly, through experimental manipulation of cues and context, rather than being able to observe the failure mode directly as it happens.
This sits inside Long-Term Memory: Encoding, Storage & Retrieval, one of seven topics in Cognitive Psychology, one of four domains in Psychology, one of seventeen subjects the app can quiz you on.